The volume of water moving through a stream channel at any given time. It integrates all upstream hydrological processes and is the most comprehensive indicator of watershed behavior.
Streamflow is the flow of water in streams and rivers, representing the integrated output of all hydrological processes occurring within a watershed including precipitation, evapotranspiration, infiltration, surface runoff, and groundwater discharge. It is typically expressed as discharge (volume per time) and is measured at stream gauging stations using stage-discharge relationships. Streamflow data are essential for virtually all aspects of water resource management, including water supply planning, flood forecasting, hydropower operations, navigation, recreation, environmental flow assessment, and water quality monitoring. The temporal pattern of streamflow, or flow regime, is characterized by annual, seasonal, and event-scale variability that reflects climate, geology, topography, and land use. Flow duration curves summarize the frequency distribution of daily streamflow, while flood frequency analyses characterize extreme events. The USGS National Water Information System (NWIS) provides real-time and historical streamflow data from thousands of gauging stations across the United States. Long-term trends in streamflow are used to assess the impacts of climate change, land use modification, and water management practices on water resources.